Various linkage arrangements are employed on vehicles to facilitate power assisted steering. One type of linkage is commonly referred to as a “parallelogram” steering linkage with a hydraulic configuration for power assisted steering. Such a linkage arrangement is commonly used on heavy duty trucks, for example.
Attempts have been made to effectively integrate electric power steering (EPS) into a “parallelogram” steering linkage architecture. Typically, a manual driver command and an electric power assist reach the linkage structure at the same location in the manner of torque. The torque is added at this location through a rack and pinion interface in an integral gear. However, the tooth pressure at this interface is significant and an adjustment mechanism may be required to periodically remove the lash at that interface. Various other components have been considered to effectively integrate EPS, such as planetary gear sets, worm and worm gears, and harmonic drives. Unfortunately, the efficiency is low for these components and the space available in the vehicle makes it impractical to place reduction mechanisms in the necessary interface location.